Why so early?

Internal reasons could be that with Xi crack down on corruption, that opponents know Xi will be gone with the term limits, and could wait that out, then go back to the corrupt norm.

Without the term limit, then the corrupt official cannot wait it out. This is like a threat without the term limit. Since they got outflanked, and when you got outflanked, you are under threat.

External reasons could be that the leadership sees real tension and a real risk of military confrontation in the coming years.

If there is a leadership change, someone might take advantage and pull some stunt in the neighbourhood, just when there is a 2 day power vacuum during the leadership change for example. Now there won't be, as leadership would be continuous like Putin.

However, if there is nothing happening, Xi could suddenly step down and pass the duties to someone else, there is flexibility to when he leaves.

A lot of unanswered questions, and unanswerable questions.